1. Field of the Invention
The invention relates to an electron beam writing method, an electron beam lithography apparatus which is called a SCALPEL apparatus, and a mask used in such an electron beam writing method and an electron beam lithography apparatus.
2. Description of the Related Art
An electron beam lithography system makes it possible to write a pattern of 0.2 xcexcm or smaller which pattern could not be written by a light-exposure system. Hence, an electron beam lithography system had drawn attention as a technology for accomplishing a smaller size and higher performance of a semiconductor device.
However, an exposure system for exposing a plurality of wafers to an electron beam at a time has not been established so far in a field of an electron beam lithography system. Hence, though an electron beam lithography system makes it possible to write a minute pattern, it was not possible to fabricate a mass of semiconductor devices at a time.
In recent years, an electron beam lithography system in which an apparatus for exposing a plurality of wafers to an electron beam at a time is used has been suggested and put into practice for the purpose of enhancement in throughput, as a semiconductor device has been designed to have a greater area and fabricated in higher integration.
Such an electron beam lithography system makes it possible to enhance fabrication yield in a large-scale integration circuit (LSI) such as a dynamic random access memory (DRAM) including a plurality of memory cells, by preparing a mask having a pattern of memory cells, and irradiating an electron beam through the mask.
The electron beam lithography system is contemplated as a substitution of a conventional light-exposure system.
However, there occurs the proximity effect in the electron beam lithography system like in the conventional light-exposure system. Herein, the proximity effect is defined as an effect wherein a pattern is influenced with respect to a size and a shape by neighboring patterns.
For instance, when a pattern of a couple of micrometers is to be formed by the electron beam lithography system, an electron beam irradiated onto a pattern is backwardly scattered due to molecules constituting a resist and a substrate, and hence, reaches an area of a resist remote from a location at which the electron beam was irradiated onto the resist, resulting in that it is not possible to form a pattern as designed. Herein, backward scattering is generally indicated as a characteristic length xcex2b.
Hence, many attempts have been made to compensate for the proximity effect by controlling electron beam irradiation such that electron energy directed to an edge of a pattern is kept constant.
One of the methods of compensating for the proximity effect includes the steps of observing neighboring patterns around patterns to be written, defining a rule table for deformation of a mask pattern in dependence on the neighboring patterns, and sizing the patterns to be written, with reference to the rule table, based on information about arrangement of a pattern. This method can deal with wide patterns of a semiconductor device.
However, this method is accompanied with a problem as follows. In this method, the mask is scanned at a constant rate as high as possible, and all patterns are exposed to a light for enhancing fabrication yield. Namely, the same amount of electron beam irradiation is applied to all patterns. Hence, even if the patterns are accurately sized, there is a limitation in writing a minute pattern with high accuracy, only by sizing the patterns.
The scanning rate may be set smaller to avoid the problem. However, there is caused another problem that fabrication yield is much deteriorated.
As mentioned so far, the conventional electron beam lithography system is accompanied with a problem that the proximity effect cannot be compensated for, even if electron beam irradiation or a scanning rate is varied. Hence, even if a minute pattern has to be formed in a local area, since the proximity effect cannot be compensated for, it would not be possible to form a pattern with high accuracy.
Apart from the above-mentioned method, various attempts have been made to compensate for the proximity effect.
For instance, Japanese Unexamined Patent Publication No. 7-106216 has suggested a method of compensating for the proximity effect, including the step of varying a scanning speed of electron beams.
In the suggested method, a deflection rate of electron beams is varied to thereby compensate for the proximity effect. However, the method is accompanied with a problem that it is unavoidable for an electron beam lithography system to have a too complicated structure.
Japanese Unexamined Patent Publication No. 8-264411 has suggested an electron beam lithography system and a method of compensating for the proximity effect in electron beam lithography.
The suggested system is designed to include a plurality of electron beam sources, an aperture, and means for overlapping electron beams relative to each other such that a current distribution of the electron beams on the aperture has a predetermined uniformity or profile.
However, this system is accompanied with a problem that it is quite difficult or almost impossible to make the current distribution to have a desired uniformity.
Japanese Unexamined Patent Publication No. 9-180978 has suggested a method of compensating for the proximity effect in electron beam lithography.
In the suggested method, there is used a mask partitioned into a plurality of small areas each having a width sufficiently smaller than a width of a profile of backwardly scattered electrons. Each of the small areas is formed with an aperture through which an electron beam passes, and is designed to have a size defined by subtracting a predetermined aperture area from an area of a non-exposed part of a wafer in a region associated with each of the small areas of the mask.
However, according to experiments conducted by the inventor, it was impossible to write a minute pattern with high accuracy by the above-mentioned method.
In view of the problems of the conventional electron beam lithography, it is an object of the present invention to provide an electron beam writing method which makes it possible to write a minute pattern with high accuracy and enhance a fabrication yield.
It is also an object of the present invention to provide an electron beam lithography apparatus which can do the same.
It is another object of the present invention to provide a mask suitable for the above-mentioned method and apparatus.
In one aspect of the present invention, there is provided an electron beam writing method including the steps of (a) preparing a mask having areas in each of which a divisional pattern is formed, the divisional pattern being obtained by dividing a pattern to be written, in accordance with an area density, and (b) irradiating an electron beam onto a wafer through the mask such that a different amount of irradiation of electron beam is applied through each of the areas of the mask.
In accordance with the above-mentioned method, it is possible to optimize electron beam irradiation for each of the divisional patterns formed in each of the areas of the mask. Hence, it would be possible to compensate for the proximity effect in each of the areas, ensuring that a minute pattern can be written with high accuracy.
In addition, by selecting an optimal scanning rate, a scanning rate might be increased in some of the areas. As a result, a fabrication yield can be enhanced in those areas.
Furthermore, the method can be carried out at low cost, and can be readily carried out in practical use.
For instance, the step (b) may be carried out by varying a rate at which a stage on which the mask is mounted is moved, or by varying a rate at which a stage on which the wafer is mounted is moved.
It is preferable that the areas are arranged in a line in the mask, and the wafer is reciprocating across each of the areas in the step (b).
Since the wafer can be exposed to electron beams while the wafer is reciprocating, it would be possible to shorten a cycle time necessary for writing a pattern, ensuring enhancement of a fabrication yield.
It is preferable that a divisional pattern in every second area among the areas is formed as a reflected image relative to an original divisional pattern, in which case, it is preferable that the wafer is moved in a direction opposite to a direction in which the mask is moved.
It is preferable that the mask is moved reciprocally within each of the areas in the step (b).
It is preferable that the areas arranged in a line have divisional patterns to be scanned in the same direction.
It is preferable that each of the areas is shaped in a square having a side length of Y/2 wherein Y indicates a backscattering diameter in a double Gaussian distribution model equation.
It is preferable that the mask has first to third areas in each of which first to third divisional patterns are formed, respectively, the first to third areas being arranged in a line in this order, the second divisional pattern being formed as a reflected image relative to an original divisional pattern, and the step (b) may further include the steps of (b1) moving the mask in a first direction and the wafer in a second direction for exposing the third area with an electron beam, the first and second directions being opposite to each other, (b2) moving the mask in the first direction and the wafer in the first direction for exposing the second area with an electron beam, and (b3) moving the mask in the first direction and the wafer in the second direction for exposing the first area with an electron beam.
It is preferable that the areas are repeatedly arranged in a group in the mask, the areas in a group having the same set of divisional patterns.
It is preferable that a divisional pattern in every second area among the areas in each group is formed as a reflected image relative to an original divisional pattern.
In another aspect of the present invention, there is provided an electron beam lithography apparatus including (a) an electron beam source emitting an electron beam, (b) a wafer stage on which a wafer is to be mounted and which is horizontally movable, (c) a horizontally movable mask located above the wafer stage, the electron beam passing through the mask and reaching the wafer, the mask having areas in each of which a divisional pattern is formed, the divisional pattern being obtained by dividing a pattern to be written, in accordance with an area density, and (d) a controller which controls a speed of at least one of the mask and the wafer stage for each of the areas in accordance with the area density.
It is preferable that the controller sets a smaller speed of at least one of the mask and the wafer stage for the area having a greater area density.
It is preferable that the areas are arranged in a line in the mask, and the controller moves the wafer reciprocally across each of the areas.
It is preferable that a divisional pattern in every second area among the areas in the mask is formed as a reflected image relative to an original divisional pattern.
It is preferable that the controller moves the mask reciprocally within each of the areas.
It is preferable that the areas arranged in a line in the mask have divisional patterns to be scanned in the same direction.
It is preferable that each of the areas in the mask is shaped in a square having a side length of Y/2 wherein Y indicates a backscattering diameter in a double Gaussian distribution model equation.
It is preferable that the controller moves the wafer stage in a direction opposite to a direction in which the mask is moved.
It is preferable that the mask has first to third areas in each of which first to third divisional patterns are formed, respectively, the first to third areas being arranged in a line in this order, the second divisional pattern being formed as a reflected image relative to an original divisional pattern, in which case, the controller (d1) moves the mask in a first direction and the wafer stage in a second direction when the third area is to be exposed with an electron beam, the first and second directions being opposite to each other, (d2) moves the mask in the first direction and the wafer stage in the first direction when the second area is to be exposed with an electron beam, and (d3) moves the mask in the first direction and the wafer stage in the second direction when the first area is to be exposed with an electron beam.
It is preferable that the areas are repeatedly arranged in a group in the mask, the areas in a group having the same set of divisional patterns.
It is preferable that a divisional pattern in every second area among the areas in each group is formed as a reflected image relative to an original divisional pattern.
In still another aspect of the present invention, there is provided a mask used in electron beam lithography, having areas in each of which a divisional pattern is formed, the divisional pattern being a pattern obtained by dividing a pattern to be written, in accordance with an area density.
In accordance with the above-mentioned mask, it is possible to optimize electron beam irradiation for each of the divisional patterns formed in each of the areas. Hence, it would be possible to compensate for the proximity effect in each of the areas, ensuring that a minute pattern can be written with high accuracy.
It is preferable that the areas are arranged in a line in the mask.
This arrangement ensures the mask to move only in a direction when a wafer is to be exposed to an electron beam through the next area of the mask. Thus, it would be possible to shorten a cycle time, ensuring enhancement of a fabrication yield.
It is preferable that a divisional pattern in every second area among the areas is formed as a reflected image relative to an original divisional pattern.
This arrangement makes it possible that the mask is successively moved in a scanning direction, and the wafer is successively reciprocating while being exposed to an electron beam. As a result, it would be possible to reduce a time necessary for the wafer to move, ensuring enhancement of a fabrication yield.
It is preferable that the areas arranged in a line have divisional patterns to be scanned in the same direction.
It is preferable that each of the areas is shaped in a square having a side length of Y/2 wherein Y indicates a backscattering diameter in a double Gaussian distribution model equation.
It is preferable that the areas are repeatedly arranged in a group in the mask, the areas in a group having the same set of divisional patterns.
It is preferable that a divisional pattern in every second area among the areas in each group is formed as a reflected image relative to an original divisional pattern.
The advantages obtained by the aforementioned present invention will be described hereinbelow.
In accordance with the present invention, it is possible to optimize electron beam irradiation for each of the divisional patterns formed in each of the areas of the mask. Hence, it would be possible to compensate for the proximity effect in each of the areas, ensuring that a minute pattern can be written with high accuracy.
In addition, the present invention makes it possible to effectively move the mask and/or the wafer, ensuring enhancement of a fabrication yield.
Furthermore, the present invention can be carried out at low cost, and can be readily carried out in practical use.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.